The invention relates to optical storage and retrieval of digital data stored as light altering characteristics on an optical material and, more particularly, to an imaging lens therefor.
Optical memories of the type having large amounts of digital data stored by light modifying characteristics of a film or thin layer of material and accessed by optical addressing without mechanical movement have been proposed but have not resulted in wide spread commercial application. The interest in such optical recording and retrieval technology is due to its projected capability of faster retrieval of large amounts of data compared to that of existing electro-optical mechanisms such as optical discs, and magnetic storage such as tape and magnetic disc, all of which require relative motion of the storage medium.
For example, in the case of optical disc memories, it is necessary to spin the record and move a read head radially to retrieve the data, which is output in serial fashion. The serial accessing of data generally requires transfer to a buffer or solid state random access memory of a data processor in order to accommodate high speed data addressing and other data operations of modern computers. Solid state ROM and RAM can provide the relatively high access speeds that are sought, but the cost, size, and heat dissipation of such devices when expanded to relatively large data capacities limit their applications.
Examples of efforts to provide the relatively large capacity storage and fast access of an optical memory of the type that is the subject of this invention are disclosed in the patent literature such as U.S. Pat. No. 3,806,643 for PHOTOGRAPHIC RECORDS OF DIGITAL INFORMATION AND PLAYBACK SYSTEMS INCLUDING OPTICAL SCANNERS and U.S. Pat. No. 3,885,094 for OPTICAL SCANNER, both by James T. Russell; U.S. Pat. No. 3,898,005 for a HIGH DENSITY OPTICAL MEMORY MEANS EMPLOYING A MULTIPLE LENS ARRAY; U.S. Pat. No. 3,996,570 for OPTICAL MASS MEMORY; U.S. Pat. No. 3,656,120 for READ-ONLY MEMORY; U.S. Pat. No. 3,676,864 for OPTICAL MEMORY APPARATUS; U.S. Pat. No. 3,899,778 for MEANS EMPLOYING A MULTIPLE LENS ARRAY FOR READING FROM A HIGH DENSITY OPTICAL STORAGE; U.S. Pat. No. 3,765,749 for OPTICAL MEMORY STORAGE AND RETRIEVAL SYSTEM; and U.S. Pat. No. 4,663,738 for HIGH DENSITY BLOCK ORIENTED SOLID STATE OPTICAL MEMORIES. While some of these systems attempt to meet the above mentioned objectives of the present invention, they fall short in one or more respects.
For example, some of the systems proposed above have lens or other optical structure not capable of providing the requisite resolution to retrieve useful data density. The optical resolution of the data image by these prior lens systems does not result in sufficient data density and data rate to compete with other forms of memory. Although certain lens systems used in other fields such as microscope objectives are theoretically capable of the needed resolutions, such lens combinations are totally unsuited for reading data stored in closely spaced data fields. Another difficulty encountered with existing designs is the practical effect of temperature and other physical disturbances of the mechanical relationship between the data film or layer, the lens assemblies and the optical sensors that convert the optical data to electrical signals. For example, the thermal expansion effects of even moderate density optical memories of this type can cause severe misregistration between the optical data image and the read out sensors. Similar difficulties are encountered in the required registration between the recording process and the subsequent reading operations. Intervening misregistration of the high density optical components can cause significant data errors if not total loss of data. Because of inherent overlap in imaging lens elements of close-pack arrays, it would be desirable to use all diffractive elements and, if possible, just a single diffractive surface. However, there are basic optical system constraints to consider. There is a defining aperture in any optical system. This aperture may be just the edges of one of the optical elements, or it (preferably, in visual devices) may be operating in a diaphragm. In optical design terms, this aperture is called the stop. The stop sets the size of the ray bundles, and it sets the location and direction of each bundle throughout the optical system. The location of the stop can have a very significant effect on image quality. For example, the aperture stop may be located at the lens. The central ray from each point in a field (such as each bit in a field of data) must pass through the center of the stop, i.e., the lens. In another example, the stop may be placed a long distance beyond the lens. The central rays still are aimed at the center of the stop, so they do not go through the center of the lens, necessarily.
In the first example, the resulting image is poor, especially for the full field bits. The reason is that all rays from all field points must pass through the same area of the same lens, hence will be acted upon in a similar way. But the ray bundles that come from a non-central field point will be going through a tilted lens, which will distort the image.
In the second example, with a lens optimized with the stop a long distance away beyond the lens and image plane, the resulting images are good for both on- and off-axis. This is because the off-axis bundles go though the lens in an asymmetric pattern, and use a partly different lens area. Since the lens can be aspheric, a large field can be accommodated.
Although the image is good for the second example, the image spot is 3/2 larger than previous lens systems using two (or more) lens elements. Therefor, other things being equal, the density of a system using a single lens will be less. Such lower density may be acceptable for certain applications.
However, there are further constraints on a single element design. In the above second example, a good image is obtained because the stop restricted the off-axis ray bundles to one side of the lens. However, this stop is an artificial construct. There can be no actual physical stop out beyond the image. In fact, because of adjacent pages, the only place to put an actual stop is at (or before) the lens. A diaphragm with a hole cannot be used at the lens, either, (because of the overlap) but the lens edges would form the stop. A stop before the lens does not work, because the bundles will be directed to the wrong side of the lens, tending to over bend the marginal rays.